Aero-optics is an interdisciplinary science combining aerodynamics and optics. For high-speed aircraft flying in the atmosphere, severe aero-optical effect will occur, which affects the imaging quality of an optical imaging detection system. Therefore, the aero-optical effect, along with its correction method, is an important research direction, and one of the main technical problems that restrict the development and application of high-speed aircrafts.
For high-speed aircraft with an optical imaging detection system flying in the atmosphere, a complex airflow field is formed by the interaction between an optical window and the airflow. Due to air viscosity, the airflow in contact with the surface of the optical window will be retarded, so that the airflow velocity decreases and a boundary layer is formed near the surface of the optical window. Within the boundary layer, the airflow layers with a large velocity gradient will have strong friction, converting kinetic energy of the airflow irreversibly into heat, and leading to rise of the temperature in the wall of the optical window. The high-temperature airflow will continuously transfer heat to the low-temperature walls, causing strong aerothermal heating. The optical window is aerothermal-heated and hence in a severe aerothermal environment; as a result, it produces thermal radiation noise, reduces signal-to-noise ratio and degrades image quality of the optoelectronic detection system.
The greater the flight speed, the more severe the aerothermal heating on the surface of the aircraft. The irradiance of the airflow outside of the optical window and the irradiance of the optical window are superimposed on the irradiance of background; as a result, an imaging sensor will enter a non-linear operation range or saturate, causing loss of effective information of scenes and reduction of signal-to-noise ratio and signal-to-clutter ratio, thus degrading the detection performance. Therefore, it is necessary to perform aerothermal radiation correction on the images acquired by the imaging sensor, in order to improve the image quality. As degradation model of aerothermal radiation is unknown and randomly changed and the degraded images contain other noise, these increase the difficulty of image restoration or correction. In addition, for specific applications in high-speed aircrafts, especially hypersonic aircrafts, the high-frame-rate characteristic of their imaging systems demands that a correction algorithm have extremely high real-time performance.